Use SHA1 cache with NOSCRIPT fallback for Redis Cluster Lua scripts

Precompute the Lua script SHA1 locally and always execute scripts via
EVALSHA to avoid repeated SCRIPT LOAD operations.

Add a robust NOSCRIPT fallback to EVAL to ensure compatibility with both
resty.redis and resty.rediscluster, especially in Redis Cluster setups
where scripts are cached per node.

This improves performance and makes script execution resilient to Redis
node restarts, failovers, and resharding.

Author: falvaradorodriguez

apisix/plugins/limit-req/util.lua

@@ -17,6 +17,7 @@
 local ngx_now           = ngx.now
 local tonumber          = tonumber
 local core              = require("apisix.core")
+local resty_string      = require("resty.string")
 
 
 local _M = {version = 0.1}
@@ -53,8 +54,24 @@ local script = core.string.compress_script([=[
   return {1, new_excess}
 ]=])
 
--- SHA1 Cache for script
-local script_sha1
+-- Pre-calculate SHA1 for EVALSHA optimization
+local script_sha1 = resty_string.to_hex(ngx.sha1_bin(script))
+
+
+local function is_noscript_error(err)
+    if not err then
+        return false
+    end
+
+    local s
+    if type(err) == "table" then
+        s = tostring(err[1] or err.err or err.message or err.msg or err)
+    else
+        s = tostring(err)
+    end
+
+    return s:find("NOSCRIPT", 1, true) ~= nil
+end
 
 
 -- the "commit" argument controls whether should we record the event in shm.
@@ -66,30 +83,18 @@ function _M.incoming(self, red, key, commit)
 
     local commit_flag = commit and "1" or "0"
 
-    local res, err
-
+    -- Try EVALSHA first (fast path).
+    local res, err = red:evalsha(script_sha1, 1, state_key,
+                                rate, now, self.burst, commit_flag)
 
-    if script_sha1 then
-        res, err = red:evalsha(script_sha1, 1, state_key,
-                               rate, now, self.burst, commit_flag)
+    -- If the script isn't cached on this Redis node, fall back to EVAL.
+    if not res and is_noscript_error(err) then
+        res, err = red:eval(script, 1, state_key,
+                            rate, now, self.burst, commit_flag)
     end
 
     if not res then
-        if not script_sha1 or (err and err:find("NOSCRIPT", 1, true)) then
-            local sha1, load_err = red:script("load", script)
-            if not sha1 then
-                return nil, "Failed to load script: " .. (load_err or "unknown error")
-            end
-            script_sha1 = sha1
-
-            res, err = red:evalsha(script_sha1, 1, state_key,
-                                   rate, now, self.burst, commit_flag)
-            if not res then
-                return nil, err
-            end
-        else
-            return nil, err
-        end
+        return nil, err
     end
 
     local allowed = tonumber(res[1]) == 1